1. Field of the Invention
The present invention relates to the lead connection with the substrate and the semiconductor element mounted thereon in semiconductor modules such as power modules mounted on electric apparatuses for automobile.
2. Description of the Related Art
Lately electronic devices have been introduced for controlling various electric apparatuses in automobiles and other vehicles. An example of electric apparatus incorporating an electronic device is an electric power steering unit shown in FIG. 14(a). In the device, a housing containing a motor relating with the steering of the automobile has a motor drive unit on which electric devices are mounted. FIG. 14(b) is a sectional view of the motor and the motor drive unit cut along a plane crossing the rotary shaft of this motor. The above-mentioned electric device is mounted on the motor drive unit as a power module.
The power module is constituted as a so-called semiconductor module with a power element such as IGBT (Insulated Gate Bipolar Transistor) suitable for controlling electric apparatuses driven by a relatively large current such as an electric power steering device mounted thereon. This type of power module is also called “an in-vehicle module” as it is mounted on vehicles.
The control unit of the above-mentioned electric power steering unit and other power modules is required to discharge efficiently the heat generated thereby from the power module to control large current with a power element (semiconductor element) mounted thereon. In order to improve the dissipation of heat from the power module, semiconductor element represented by MOS (Metal-Oxide Semiconductor) are connected by die bonding with the power module in the form of bare chips (not sealed by resin and the like) on the substrate,
As the form of electrical connection with semiconductor element mounted on the substrate of power modules, the A1 wire bonding connection method used in other semiconductor modules has been used in the past. FIG. 1 shows an example of chip connection in the prior power modules. FIG. 1 is a sectional view showing conceptually by enlarging the part related with the chip connection of the power module, and the substrate (metal base substrate) 1 includes a metal base 1a, an insulation layer 1b covering the main surface of a side of the metal base 1a, and a wiring 1c formed by metal and other conductors on the insulation layer 1b (pad on the substrate side 1d). One side of the wiring 1c is connected by solder 2 with the lower surface of the semiconductor element 3 (one side of the main surface), and the upper surface of the semiconductor ship 3 (the other side of the main surface, opposite side of the main surface of the one side) and the other side of the wiring is electrically connected with A1 wire. Incidentally, the system of coordinates shown in FIG. 1 specifies the main surface of the substrate 1 as x-y plane and defines the positional relationship between the structure shown in FIG. 1 and the structures in other figures described below. The x axis, the y axis and the z axis in this system of coordinates need not cross each other at right angle as long as they meet the condition of crossing each other.
As the bonding wire 4 used for connecting chips by A1 wire bonding shown in FIG. 1 is slender, the resistance against current controlled by the chip necessarily grows higher. In order to avoid the rise in electric resistance in such chip connections, a plurality of wires must be connected to a MOS chip. And for mass producing power modules wherein such a chip connection method is adopted, the number of wire bonders and other similar equipment must be increased, and if the production of such power modules is to be increased, new capital investments will be required. Therefore, it is desired to realize chip connections in power modules not by wire bonding but by soldering leads producing a low resistance and suitable for the mass production of the power modules. FIG. 2 shows an example of power modules whose chips are connected with the lead. The upper surface of the semiconductor element 3 and the wiring 1c at the other end are respectively connected by solder 2 to the lead 6. And a heat spreader 5 is provided between the wiring 1c on the other end mentioned above and the lower surface of the semiconductor chip 3.
In order to realize such a lead connection of such semiconductor modules, the following technology is now under consideration. For example, Patent Document 1 teaches the method of connecting in layers a lead, semiconductor element, and a heat sink on the substrate by using a plurality of solders with different melting points, and that of connecting them in one stroke by using a Sn solder paste (containing mainly tin). This method simplifies the assembly process that will be necessarily complex by the wire bonding connection method. And Patent Document 2 examines how to improve the reliability of soldering by using a heat sink, an insulating substrate, silicon chips and a material with a linear expansion coefficient difference of 10×10−6 or less. And Non-Patent Document 1 examines how to reduce thermal stress applied by the lead to the chips and to prevent the breakdown of chip electrodes by making the ends of the lead comb-teeth shaped.
[Patent Document 2]
[Non-patent Document 1]
The patent documents each referred above or later are listed as follows.    [Patent Document 1] Japanese Patent Publication Gazette No. 3627591    [Patent Document 2] Japanese Unexamined Patent Publication No. hei 9-36186 (JP-A No. 36186-1997)    [Non-Patent Document 1] Nakajima et al.: “Lead connected type high-capacity resin-sealed power module”, 11th Symposium on “Microjoining and Assembly Technology in Electronics” (Mate 2005), Vol. 11 (2005), pp. 433-436.